Finding acceleration and force of pulling

  • Thread starter Thread starter Whitishcube
  • Start date Start date
  • Tags Tags
    Acceleration Force
Click For Summary
SUMMARY

The discussion focuses on calculating the force required for a window washer to pull herself up using a pulley system. The combined mass of the washer and bucket is 72 kg. To lift at a constant speed, the washer must exert a downward force of 705.6 N. When this force is increased by 15%, resulting in a force of 811.44 N, the calculated acceleration is 0.74 m/s². The solution involves applying Newton's laws and free body diagrams to analyze the forces acting on the system.

PREREQUISITES
  • Understanding of Newton's laws of motion
  • Ability to construct free body diagrams (FBD)
  • Familiarity with basic physics equations, particularly F_net = ma
  • Knowledge of tension forces in pulley systems
NEXT STEPS
  • Study the principles of tension in pulley systems
  • Learn how to apply Newton's second law in various scenarios
  • Explore advanced free body diagram techniques for complex systems
  • Investigate the effects of varying mass and force on acceleration
USEFUL FOR

Physics students, educators, and anyone interested in understanding mechanics, particularly in relation to forces and motion in pulley systems.

Whitishcube
Messages
83
Reaction score
0

Homework Statement


a window washer pulls herself up using a single pulley by pulling on the rope tied to the bucket she is in. the combined mass of the bucket and the washer is 72kg. a)how hard must she pull downward to be lifted at a constant speed? b) if this force is increased by 15%, what will her acceleration be?


Homework Equations



F_net= ma

The Attempt at a Solution


so part a was pretty easy. after setting up two freebody diagrams, i have
F_(net person)=F_tension - mg = ma
and
F_(net of hand pulling)= F_t - F_Pull = Ma?

one problem i had was knowing what the second freebody diagram is... i know the first one is the washer in the bucket, but I am not sure if the second one is the hand pulling on the rope (where the hand is the object), or if its the rope itself, in which case the force of the hand is what's pulling down and the tension upwards.
either way, i solved both equations for F_t (force of tension) by setting both of them equal to zero (due to constant velocity meaning 0 acceleration) and solved for the F_pull (the force of the person pulling).
so i get: F_pull = 705.6 N

for part b, i multiply the F_p by 1.15 for the 15% extra force. this gives me F_p=811.44 N.
from there i set up my two free body diagrams again and got:
F_t - mg = ma
and
F_t - F_p = M(-a)

plugging in 72kg for m gives F_t = 72a + 705.6.
the second part confuses me again. I am not sure what the mass would be in the second case. so i used the mass of the person (72kg) as M and solved for a.
on a side note, the acceleration in the second equation is negative because of the way i chose my axes and because of the direction of the acceleration.

then i plugged in F_t into the other equation, yielding:
72a + 705.6 - 811.44 = -72a
solving for a gives: a = 0.74 m/s^2

Any advice that could be given on this type of problem would be great. usually i could do this if i were given some other object that was tied to the rope instead of the person pulling on it. Thanks
 
Physics news on Phys.org
nevermind. i figured it out a much simpler way
 
You can't draw a FBD of the person without including the unknown normal force acting on the person from the bucket. Instead, draw a FBD that cuts through both ropes and encircles the bucket with person in it, then use Newton 1 for part a.

Edit, oh, nevermind, I see you got it, good.
 
Last edited:

Similar threads

  • · Replies 4 ·
Replies
4
Views
3K
  • · Replies 38 ·
2
Replies
38
Views
5K
Replies
6
Views
4K
  • · Replies 4 ·
Replies
4
Views
3K
  • · Replies 7 ·
Replies
7
Views
3K
Replies
4
Views
1K
  • · Replies 9 ·
Replies
9
Views
9K
Replies
11
Views
1K
  • · Replies 22 ·
Replies
22
Views
2K